1. Field of the Invention
The present invention relates to a developing roller for use in an electrophotographic image-forming apparatus adopting an electrophotographic mode such as a copying machine, a printer, or the receiving apparatus of a facsimile, and a method of producing the roller.
In addition, the present invention relates to a process cartridge and an electrophotographic image-forming apparatus each of which uses the developing roller.
2. Description of the Related Art
A contact developing method has been known as a method of developing an image in an electrophotographic apparatus. In the contact developing method, an electrostatic latent image formed on a photosensitive drum is carried by the surface of a developing roller abutting the photosensitive drum, and is developed with a developer conveyed to a developing zone.
A developing apparatus to be used in such developing method includes a developer container storing the developer and the developing roller. Further, the apparatus has: a developer supplying roller for supplying the developer in the developer container to the developing roller, the developer supplying roller being placed so as to abut the developing roller; and a developing blade which forms a thin film of the developer on the surface of the developing roller and makes the amount of the developer on the surface of the developing roller constant.
The surface of the developing roller is rubbed with the developing blade. Accordingly, high toughness is requested of the surface of the developing roller. When the surface of the developing roller has poor toughness, the surface of the developing roller is shaved upon long-term use of the developing roller, with the result that an image failure occurs in some cases. Meanwhile, such softness that the developer is not squashed excessively is requested of the surface of the developing roller. When the surface of the developing roller is hard, the developer is squashed with the developing roller, so the melt adhesion of the developer to the surface of the developing roller (filming) may occur upon long-term use of the developing roller. In addition, the filming may cause fog in an image. By the foregoing reasons, a polyurethane resin providing a high-toughness, soft surface layer has been frequently used as a component for the surface layer of the developing roller.
Japanese Patent Application Laid-Open No. 2006-251342 relates to a developing roll including a base rubber layer and a surface layer provided on the base rubber layer and containing a polyurethane resin. In addition, the document discloses the following invention: the surface layer is formed of a resin composition containing a specific polyether group polyol, a specific diisocyanate, and a specific aromatic group two-functional chain extending agent and free of any electron conductive agent so that the resistance of the surface layer to the adhesion of low melting point toner may be improved.
In addition, Japanese Patent Application Laid-Open No. 2005-141192 relates to a developing roller having a conductive elastic layer and a conductive surface layer provided on the conductive elastic layer and containing a polyurethane resin. The document discloses the following invention: the conductive surface layer is formed of a urethane raw material formed of a specific polyurethane polyol prepolymer and a specific isocyanate compound so that a reduction in image density under a low-temperature, low-humidity environment and the peeling of the conductive surface layer under a high-temperature, high-humidity environment may be prevented.
By the way, in recent years, the following ability, which has not been conventionally requested, has been requested of a developing roller including such surface layer containing a polyurethane resin in the field of electrophotography: the roller can exert stable performance even under an extremely severe environment. That is, in an unused process cartridge, a developing roller and a developing blade contact each other at all times with a developer interposed between them in order that the developing blade may be prevented from sticking to the developing roller during the storage of the process cartridge. However, when the process cartridge in such state is left to stand under a high-temperature, high-humidity environment having a temperature of 40° C. and a humidity of 95% RH for a long time period, the developer interposed at the abutting portion of the developing roller and the developing blade often adheres to the surface of the developing roller. The adhering developer continues to adhere to the surface of the developing roller even after the following state: the process cartridge is mounted on an electrophotographic image-forming apparatus so as to be put into use for the formation of an electrophotographic image. As a result, a stripe-like defect called banding often occurs in the electrophotographic image. Such defect can occur in a particularly remarkable fashion in a halftone image. Meanwhile, the developing roller has currently been requested to suppress fog resulting from the filming of the developer which may occur upon formation of an electrophotographic image under a low-temperature, low-humidity environment having a temperature of 10° C. and a humidity of 14% RH (hereinafter simply referred to as “fog”).